1. Field of the Invention
The present invention relates to a method for forming a thin film of oxide complex compound (complex compound, hereinafter referred as complex compound), more specifically, a method for forming complex compound layer by utilizing sol-gel method.
2. Description of the Prior Art
In general, vacuum deposition methods or sputtering methods are commonly used for forming a functional film (layer) which is applied for electric devices. However, these method have difficulty in controlling the formation of the layer and reproducibility at ferroelectric layer of multi-ingredients, superconducting layer and conducting layer. Also, similar difficulty is observed when forming the layer under low temperature condition as well as forming the layer to large area.
Recently, to resolve above issues, a sol-gel method is often used as one technique for forming a thin film of complex compound.
FIG. 1 shows a typical steps of forming a thin film of complex compound under sol-gel method of the prior art. With the sol-gel method, the thin film is formed on a substrate of a semiconductor device through following three steps such as coating sol solution on the substrate, dehydration of the sol solution and baking the sol solution on the substrate. Initially, adequate quantity of water is added to composite alkoxide (for instance, PZT (Pb (lead), Zr(zirconium) and Ti(titanium)) to cause hydrolysis and polycondensation as shown in step "a" of FIG. 1. Thus, sol solution is prepared as shown in "b" of FIG. 1. The sol solution is coated on the substrate of a semiconductor device to obtain a layer of polymer gel state by dipping coat method or spin coat method (see step "c" of FIG. 1). Dipping coat method is a method for obtaining the layer on the substrate, that is the substrate is dipped into the sol solution thoroughly, then the substrate is pulled out from the sol solution in constant speed. On the other hand, the spin coat method is a method for obtaining uniform layer on the substrate, that is the solution is dropped onto the substrate, then the substrate is rotated at a high speed.
The layer is dehydrated on a hot plate at 100.degree. C. to 450.degree. C. (see step "d" of FIG. 1), then the layer is baked at 650.degree. C. These processes forms a PZT oxide thin film as shown in step "e" of FIG. 1. Thus, a thin film of complex compound is formed on a surface of the substrate of a semiconductor device.
However, above described method for forming a thin film of complex compound has following problems.
In the sol-gel method, to obtain a layer which has thickness over 100 nanometers (nm), some cracks are caused as shown in FIG. 2 from shrinkage of the layer at thermal treatment (dehydration, baking) process. Therefore, the thickness of the layer is limited to nearly 20 to 100 nm, which can be formed in single thermal treatment such as coating the sol solution, dehydration of the solution, dehydration and baking, due to shrinkage of the thin film during the thermal treatment.
From this fact, repeated thermal treatments such as coating, dehydration, baking process are necessary to achieve reliable insulation.